Partially Cystic Frontal Nerve Schwannoma Masquerading as Abducens Nerve Paresis

A 62-year-old male presented with a 1-year history of horizontal diplopia that was worse in left gaze. He was initially diagnosed with left abducens nerve paresis by a strabismus specialist. The strabismus specialist noted −1 abduction deficit of the left eye and 8 PD of esotropia at primary gaze. He additionally had 1.5 mm of relative exophthalmos of the left eye. An MRI of the orbits is shown in Figure 1. T1W coronal MRI identifies a heterogeneous, cystic-appearing anterior portion of a mass lesion superior to the superior rectus (Fig. 1A, arrow). T1W coronal MRI of the orbital apex shows a more homogenous, hyperintense portion of the tumor (Fig. 1B). T2W coronal MRI of the anterior portion of the lesion shows a homogenous hyperintensity, as often seen in cystic structures (Fig. 1C). T1W sagittal MRI with contrast depicts the posterior aspect of the tumor in the orbital apex (Fig. 1D, arrow). A craniotomy-approached orbitotomy was performed to completely excise the tumor. Pathological analysis revealed that the mass was a schwannoma. The origin was thought to be the frontal nerve based on its location in the orbit. Postoperatively, the patient’s diplopia completely resolved with almost complete resolution of his abduction deficit. Based on the tumor’s location outside both the muscle cone and cavernous sinus, the abduction deficit may have been caused by the tumor’s effect on the orbital fascial and pulley system with mechanical restriction or possibly by mechanical distortion of the abducens nerve or lateral rectus at the orbital apex.

FIG. 1.: A. T1W coronal MRI demonstrating a heterogeneous, cystic lesion superior to the superior rectus muscle (arrow). B. A more posterior cut of the T1W coronal MRI shows a hyperintense lesion near at the orbital apex. C. T2W coronal MRI shows a hyperintense lesion at approximately the same level as Figure 1A, which is consistent with the cystic property of the lesion at this location. D. T1W sagittal MRI with gadolinium contrast demonstrates this mass lesion along the superior posterior orbit (arrow).